Selective Adsorption of Oxygen from Humid Air in a Metal–Organic Framework with Trigonal Pyramidal Copper(I) Sites

Abstract

High or enriched-purity O₂ is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O₂-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O₂ over N₂ <i>via</i> electron transfer. However, most of these materials exhibit appreciable and/or reversible O₂ uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework Cu^I-MFU-4<i>l</i> (Cu_<i>x</i>Zn_5-<i>x</i>Cl_4-<i>x</i>(btdd)₃; H₂btdd = bis(1<i>H</i>-1,2,3-triazolo[4,5-<i>b</i>],[4',5'-<i>i</i>])dibenzo[1,4]dioxin), which binds O₂ reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal Cu^I sites, and we show that this material reversibly captures O₂ from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, Cu^I-MFU-4<i>l</i> retains a constant O₂ capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N₂, differences in O₂ and N₂ desorption kinetics allow for the isolation of high-purity O₂ (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O₂ binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that Cu^I-MFU-4<i>l</i> is a promising material for the separation of O₂ from ambient air, even without dehumidification.